Question

The unit of the power of the lens is:

• A. metre
• B. per metre
• C. metre per second
• D. none of these

Hint:

The power of a lens is measured in diopters, which is equivalent to per meter.

Answer 1

The Correct Option is B

The power of a lens is defined as the reciprocal of its focal length: \[ P = \frac{1}{f} \] where \( P \) is the power of the lens, and \( f \) is the focal length of the lens.
Step 1: Units of focal length.
The unit of focal length \( f \) is meter (m).
Step 2: Units of power.
Since power \( P \) is the reciprocal of focal length, its unit will be the reciprocal of the unit of focal length: \[ \text{Unit of } P = \frac{1}{\text{Unit of } f} = \frac{1}{\text{meter}} = \text{per meter} \]
Step 3: Conclusion.
Thus, the unit of the power of the lens is per meter. The correct answer is (B).